Why OSI is called Open System?

Answer 1

The Open Systems Interface was developed so that various computers could talk to one another. It was the foundation for basic standards that finally lead to standardized network technologies.

Before that, IBM computers could only talk with IBM computer using IBM software and IBM hardware. Unisys, CDC, etc etc all had their own standards too.

Now, you have everyone pretty much standardized on Ethernet hardware using the TPC/IP protocol. Instead of spending $200 per network card, you spend $20 (if that), and everyone is talking to everyone.